1. Field of the Invention
The invention relates to radar antenna systems and, more particularly, to rotating RF couplings to accommodate antenna mechanical rotation.
2. Description of the Prior Art
In the prior art, rotating RF couplings have been provided in a number of forms. A brief summary of these devices is contained in the test Radar Handbook by Merrill I. Skolnik (McGraw-Hill 1970) in Chapter 8 under a paragraph entitled "Rotary Joints." Bibliographical references concerning the theory and design of rotary joints are included with that summary.
The patent literature also includes description of prior art devices of conventional type, such as the apparatus of U.S. Pat. No. 2,751,559, for example.
In U.S. Pat. No. 3,896,446, a rotating RF joint is involved in a scanning radar system in which the antenna system is mounted over the top of a helicopter rotor to rotate with it; however, no advances in the rotary joint itself appear to be disclosed.
Other rotary RF joints, such as shown in U.S. Pat. No. 3,123,782 appear to be based on conductive circular ring arrangements and as such may be thought of as "slip-ring" devices.
Still further, systems not including sliding contacts but providing electromagnetic energy transfer in a rotating joint include multihorn configurations and the like. U.S. Pat. No. 3,803,619, and also U.S. Pat. Nos. 3,117,291 and 3,108,235 disclose devices of that type.
The slip-ring systems are known to suffer from arcing, mechanical wear, and other problems, and the rotating horn devices are complex and costly and leave much to be desired in energy transfer efficiency.
In the most familar shipboard rotating antenna installations, lateral structural members affixed to mast structures have been required to provide horizontal clearance for a rotating antenna system. Prior art rotary joints of the aforementioned and other conventional types have been applied in such instances. Many have used such expedients as circular waveguide, coaxial transmission line sections and the like providing rotatable conductive walls essentially concentric with the axis of rotation of the antenna structure itself.
A device is described in U.S. Pat. No. 4,222,055 (assigned to the assignee of this application) utilizes a pair of facing annular ring-like subassemblies each divided into cells producing a series of circumferentially disposed open-ended, facing, waveguide sections. The cross-sections of these waveguides are in planes normal to the common axis of the two annular subassemblies. One of these rings is fixed (stator) and the other rotates (rotor) in a fixed mechanical relationship with a rotating antenna array. The interface between rotor and stator facilitates energy transfer, the stator waveguide cells all being dimensionally equal and equally excited to provide a uniform energy distribution pattern over the circumference of the interface.
It is also pointed out in U.S. Pat. No. 4,222,055 that a taper of excitation applied to the driven array elements or columns of elements can be provided by variation of rotor cell sizes.
In the course of providing the desired circumferentially uniform excitation at the rotor/stator interface, the device of the aforementioned U.S. Pat. No. 4,222,055 employs a predetermined number of waveguide cells N, each having a circumferential dimension .theta., where .theta.=360/N. Each cell is fed from a transmission line tap, the line being fed at one end from a source (transmitter, etc.) and terminated in a resistive load at its other end.
The termination load acts to absorb power not taken by the taps, and furthermore it can be shown that some power will always be committed to the load. A direct connection to a "last" stator cell is possible, but in the configuration contemplated in the aforementioned U.S. Pat. No. 4,222,055, the power supplied to this last cell would be unequal to that extant in the remaining cells, electric field non-uniformity at the rotor/stator interface resulting. The uniformity of the circumferential electric field at the rotor/stator interface is important and in view of that, the load terminated line was employed in this prior art configuration. The use of a 3 db coupler at the "last" stator cell is theoretically possible to avoid use of the load; however, the implementation of a 3 db coupler has been found to be technically un-realizable.
A typical prior art arrangement comprised twenty stator cells each taking approximately 4.65% of the total line input power, the remaining 7% of the power being absorbed in the termination.
The obvious loss of power efficiency accepted for the sake of circumferential uniformity of the electric field at the rotor/stator interface is undesirable, and the manner in which this loss is eliminated according to the invention will be understood as this description proceeds.